Geoscience Reference
In-Depth Information
8.13 Hydrothermal Synthesis of Arsenates
Arsenates are closer to phosphates and vanadates in their mineralogy and crystal
chemistry. There are several arsenate minerals occurring in nature, like mimetite,
(PbCl) Pb
4
(AsO
4
)
3
, tilasite (MgF) CaAsO
4
; adelite [(MgOH)CaAsO
4
], sarkinite
[(MnOH) MnAsO
4
], olivenite [Cu
2
(OH) AsO
4
], adamite [Zn
2
(OH)AsO
4
], clinocla-
site (6CuO
2H
2
O. However, the
number of arsenate minerals is much less than the number of vanadates and phos-
phates. The same is the case with reference to the synthetic analogs, i.e., the
number of synthetic arsenates is much less than the number of synthetic phosphates
and vanadates. Hence, in comparison with the rich structural chemistry of transition
metal phosphates, reports on the arsenates are rare. Although a few arsenates have
stoichiometries similar to those of the phosphate analogs, their structures are con-
siderably different. Most of these arsenates adopt new structural types and have
stoichiometries not found in the phosphates. The hydrothermal technique helps
immensely to obtain new structures among arsenates. Some of the arsenates synthe-
sized belong to the NASICON family Na
As
2
O
3
3H
2
O), soselite (Ca,Co,Mg)
3
, and As
2
O
8
Zr
2
(AsO
4
)
3
;A
2
1
B
2
1
As
2
O
7
(A
5
Ca, Sr,
Ba; B
Cu), both exhibiting superionic properties, NaNb
2
AsO
8
with nonlinear
optical properties, ABXO
4
type [A
5
Mn, Zn, Cu
2
1
,Co
2
1
,Mg
2
1
, etc.; B
5
5
(OH);
X
As], several complex arsenates showing unique structures related to zeolites.
In spite of a wide structural diversity, the hydrothermal synthesis of arsenates is not
very popular owing to the best known reasons, that arsenic is highly corrosive and
highly reaction susceptible. Also, it contaminates the system unless the experiments
are carried out at T
,
5
300
C using Teflon liners
[129
133]
.
The majority of experiments in recent years have been carried out in stainless
steel autoclaves provided with Teflon lining. However, the high-temperature
experiments, like in the case of NaNb
2
AsO
8
, are carried out in sealed gold liners,
at T
750
C and P
32,000 psi.
For example, the copper diarsenates (ACuAs
2
O
7
,A
5
5
Ca, Sr, Ba) synthesis has
been carried out in Teflon-lined autoclaves at 230
C for 4 days, followed by a slow
cooling to room temperature at 50
C/h. The pure phase is obtained by heating a
mixture of Me
2
1
(OH)
2
(Me
2
1
5
5
Ca, Sr, Ba), CuO, KCl, H
3
AsO
4
, pyridine, and
water, in the following molar ratios:
Ca
2
1
:Cu:As
5
1:2:7.5
Sr
2
1
:Cu:As
5
1:4:15
Ba
2
1
:Cu:As
5
1:2:7.5
The crystal chemical study of these A
2
1
B
2
1
As
2
O
7
compounds shows that the
As
2
O
7
group is very adaptive to the bonding requirements of other groups in the
structure by adjusting the As
As bond angle and the torsional angle between
the two AsO
4
tetrahedra. The size of the alkali earth metal cation seems not to play
an important role in deciding the structural types of the arsenates as it does on the
corresponding phosphates.
The complex arsenates, like (Mg, Ni)
2
(OH)(AsO
4
), could be prepared using
hydrothermal conditions and starting from the (Mg
3
2
x
Ni
x
) (AsO
4
)
2
a
O
a
8H
2
O(X
0,
5
